1. Field of Invention
This invention relates to target detection systems. Specifically, the present invention relates to systems employing electro-optical sensors to detect targets using constant false alarm rate detection processes.
2. Description of the Related Art
Target detection systems are used in a variety of demanding applications including radar air traffic control systems, missile target tracking systems, and electro-optical target detection systems employed on aircraft and ground-based military vehicles. Such applications often require accurate target detection systems that produce minimal false detections.
A target detection system typically includes an electromagnetic energy sensor that receives electromagnetic signals such as optical signals and outputs electronic signals in response thereto. A processing circuit analyzes the electronic output signals to determine if a target is present in the field of view of the sensor.
The sensor is often a focal plane array of electromagnetic energy detectors such as charge-coupled devices (CCDs). Detectors in the array may have different performance characteristics that may change with a changing signal environment. Often, the detectors are initially calibrated by aiming the sensor at a dark, uniform region of space. Electrical offset values or gain coefficients are applied to the outputs of the detectors to equalize the outputs and thereby compensate for detector signal non-uniformities.
In a typical constant false alarm rate (CFAR) target detection system, the processing circuit includes a detector non-uniformity correction circuit for performing the calibration, a background estimation circuit, and a threshold circuit. The background estimation circuit determines an initial background value that is subtracted from the outputs of the detectors to enhance signal-to-noise ratio. The threshold circuit establishes a detection voltage threshold range for the detectors in the array. Typically, a single threshold range is established for all detectors in the array.
An `alarm` occurs when the magnitude of a detector output signal is within the threshold range. By controlling the threshold range, the target detection system can control the probability of making a false detection. However, decreasing the probability of false detection may increase the likelihood that a target will go undetected.
Use of a single threshold range for all detectors in the array is inefficient, as the performance capabilities of individual detectors are often not maximized. For example, low performance detectors may raise the desired lower threshold of the threshold range. Due to the higher threshold, the capability of any high performance pixels to detect targets in noisy environments is not utilized.
Detector background estimation is often performed when the target detection system is initially activated and is disabled thereafter. In existing systems, if the background estimation circuit remains enabled, target information may corrupt the background estimates. The corrupted values may greatly reduce the target detection capability of the system. Accordingly, many existing target detection systems fail to account for variations in background that often occur during system operation. As a result, the ability of such target detection systems to accurately detect targets is compromised.
Hence, a need exists in the art for an accurate target detection system that accounts for varying detector background levels and changing signal environments during system operation.